1. Field of the Invention
The present invention relates to a method of making a flexographic printing master by inkjet.
2. Description of the Related Art
Flexography is today one of the most important printing techniques and is commonly used for high-volume runs. Flexography is used for printing on a variety of substrates such as paper, paperboard stock, corrugated board, films, foils and laminates. Coarse surfaces and stretch films can only be economically printed with flexography, making it indeed very appropriate for packaging material printing.
Today flexographic printing masters are prepared by both analogue and digital imaging techniques. Analogue imaging typically uses a film mask through which a flexographic printing precursor is exposed. Digital imaging techniques include:                Direct laser engraving as disclosed in e.g. EP-As 1710093 and 1936438;        UV exposure through a LAMS mask wherein LAMS stands for Laser Ablative Mask System as disclosed in e.g. EP-A 1170121;        Direct UV or violet exposure by laser or LED as disclosed in e.g. U.S. Pat. No. 6,806,018; and        Inkjet printing as disclosed in e.g. EP-As 1428666 and 1637322.        
EP-A 1428666 discloses a method of making a flexographic printing master by means of jetting subsequent layers of a curable fluid on a flexographic support. Before jetting the following layer, the previous layer is immobilized by a curing step.
U.S. Pat. No. 6,520,084 also discloses a method of preparing flexographic printing masters using inkjet. In this method, a removable filler material is used to support the relief image being printed and the relief image is grown in inverted orientation on a substrate. Disadvantages of this method are the removal of the filler material and the release of the relief image from the substrate. In U.S. Pat. No. 7,036,430 a flexographic printing master is prepared by inkjet wherein each layer of ink is first jetted and partially cured on a blanket whereupon each such layer is then transferred to a substrate having an elastomeric floor, thereby building up the relief image layer by layer. US20080053326 discloses a method of making a flexographic printing master by inkjet wherein successive layers of a polymer are applied to a specific optimized substrate. In US20090197013, also disclosing an inkjet method of making a flexographic printing master, curing means are provided to additionally cure, for example, the side surfaces of the image relief being formed.
The major advantage of an inkjet method for preparing a flexographic printing master is an improved sustainability due to the absence of any processing steps and the consumption of no more material as necessary to form a suitable relief image (i.e. removal of material in the non printing areas is no longer required).
A problem however that may occur with these inkjet methods is the lack of smoothness of the at least partially cured layers of fluid. Such lack of smoothness may be passed on from layer to layer forming the relief image or may even be reinforced as more layers are jetted on top each other and may result in an unsmooth printing surface of the relief, which can give rise to several printing artifacts such as a deficient reproduction of highlight dots or a deficient rendering of solids. For optimal printing performance, it is required that flexographic printing masters have a relief image with a printing surface that is sufficiently flat or even.